Combustion control device for gas engine

ABSTRACT

An object is to prevent an engine from entering an unstable combustion state such as misfire upon recovery, by halting or reducing gas fuel for a cylinder in which knocking is occurring, and then increasing the gas fuel again to appropriately perform a recovery control upon recovery to the optimum operation. A combustion control device for a gas engine includes: a knocking determination unit  49  configured to determine occurrence of knocking of each of cylinders; a knocking reduction unit  55  configured to halt or reduce supply of gas fuel to a cylinder in which the knocking is occurring and reduce supply of the gas fuel to other cylinders in which the knocking is not occurring; a first recovery unit  57  configured to recover a state where the gas fuel is halted or reduced in the cylinder in which the knocking is occurring; and a second recovery unit  59  configured to recover a state where the gas fuel is reduced in the other cylinders which are not the cylinder in which the knocking is occurring. A recovery time of the first recovery unit  57  is shorter than a recovery time of the second recovery unit  59,  and recovery of the cylinder in which the knocking is occurring is performed as a priority.

TECHNICAL FIELD

The present invention relates to a combustion control device for a gasengine, and to a combustion control device with respect to knocking inparticular.

BACKGROUND

Power generation facilities operated by a gas engine using natural gasor city gas as main fuel are launched in view of clean energy source.

To make a gas engine perform stable and efficient operation, fuel supplyvalves provided for respective cylinders are controlled, and theignition timing is also controlled. However, abnormal combustion such asknocking and misfire may still occur. It is necessary to detect andavoid abnormal combustion such as knocking and misfire at an earlystage.

To avoid knocking, supply of gas fuel is halted or reduced for acylinder in which knocking has occurred, or the ignition timing isretarded in some cases. For instance, Patent Document 1 (JP4688916B) andPatent Document 2 (JP4247842B) are known.

Patent Document 1 discloses a load leveling control in which an exhausttemperature is detected for each cylinder of a gas engine, a cylinder inwhich knocking or misfire has appeared is detected, then an amount offuel supply to a cylinder having the maximum exhaust temperature isreduced, and an amount of fuel supply to a cylinder having the minimumexhaust temperature is increased. Further, supply of fuel is halted orreduced for a predetermined period of time for the cylinder in whichknocking or misfire has occurred. Furthermore, the load leveling controlis performed excluding the cylinder for which a measure for addressingknocking or misfire is being taken.

Further, Patent Document 2 discloses a knocking control device for a gasengine. The magnitude of knocking is detected by a knocking sensor, andthe mean value of occurrence frequency in each cylinder is compared withthe occurrence frequency in a certain cylinder on the basis of ameasurement value of occurrence frequency of knocking that is apredetermined value or more. If the occurrence frequency in the certaincylinder is not less than a predetermined frequency as compared to themean value of occurrence frequency, the amount of gas injection isreduced for the certain cylinder. If the occurrence frequency in thecylinder is not more than a predetermined frequency as compared to themean value of occurrence frequency, the amount of gas injection isincreased for the cylinder.

Further, Patent Document 2 similarly discloses retarding and advancingthe injection timing of the pilot fuel injection timing and the ignitiontiming of the spark ignition timing.

CITATION LIST Patent Literature

Patent Document 1: JP4688916B

Patent Document 2: JP4247842B

SUMMARY Problems to be Solved

Patent Documents 1 and 2 both disclose halting or reducing supply of gasfuel or retarding the ignition timing for a cylinder in which knockinghas occurred, as a countermeasure technique to knocking or misfire, butdo not go as far as disclosing recovery control in which the amount ofgas fuel is increased or the injection timing or the ignition timing ofgas fuel is recovered again after performing a countermeasure control.

Further, in case the above recovery control is not appropriatelyperformed, misfire or knocking may occur again upon recovery.Especially, the temperature in the combustion chamber decreases due tohalting or reducing supply of gas fuel as a countermeasure to knocking.Thus, if the amount of gas fuel is merely increased upon the recoverycontrol of the gas fuel, abnormal combustion such as misfire may occuragain upon recovery.

The present invention was made in view of the above technical issue. Anobject is to provide a combustion control device for a gas engine whichprevents an unstable combustion state such as misfire upon recovery byappropriately performing recovery control for recovering to the optimumoperation by increasing gas fuel again after halt or reduction of thegas fuel for a cylinder in which knocking has occurred as acountermeasure to knocking in the gas engine.

Solution to Problems

The present invention was made to achieve the above object, and providesa combustion control device for a gas engine, including: a knockingdetermination unit configured to determine occurrence of knocking ofeach of cylinders; a knocking reduction unit configured to halt orreduce supply of gas fuel to a cylinder in which the knocking isoccurring and reduce supply of the gas fuel to other cylinders in whichthe knocking is not occurring, when the occurrence of the knocking isdetermined by the knocking occurrence determination unit; a firstrecovery unit configured to recover a state where the gas fuel is haltedor reduced in the cylinder in which the knocking is occurring, when itis determined that the knocking is not occurring after the halt orreduction; and a second recovery unit configured to recover a statewhere the gas fuel is reduced in the other cylinders which are not thecylinder in which the knocking is occurring. A recovery time of thefirst recovery unit is shorter than a recovery time of the secondrecovery unit, and recovery of the cylinder in which the knocking isoccurring is performed as a priority.

According to the present invention, when it is determined that knockingis occurring, supply of gas fuel to the cylinder in which knocking isoccurring is halted or reduced, and supply of gas fuel to cylindersother than the cylinder in which knocking is occurring is reduced.

In this way, the air-fuel ratio with respect to the cylinder in whichknocking is occurring shifts to the lean side, and thus occurrence ofknocking is restricted. Further, also in the other cylinders in whichthe knocking is not occurring, the gas fuel is reduced in response to apredetermined command to reduce the load due to a decrease in the powergeneration output to prevent overload.

Such reduction or halt of the gas fuel restricts occurrence of knocking.Later, when it is determined that knocking is not occurring by theknocking determination unit, the reduced or halted supply of gas fuel isrecovered and returned to the original condition, i.e., the requiredamount of gas fuel corresponding to the required load.

The first recovery unit recovers the cylinder in which knocking isoccurring from a state in which the gas fuel is halted or reduced, andthe second recovery unit recovers cylinders other than the cylinder inwhich knocking is occurring from a state in which the gas fuel isreduced. Then, the recovery time of the first recovery unit is set to beshorter than the recovery time of the second recovery unit, so thatrecovery of the cylinder in which knocking is occurring is performed inpriority.

Thus, recovery is firstly performed for the cylinder in which knockingis occurring, and then if abnormality is not occurring again, i.e.,after confirming that abnormal combustion such as knocking and misfireis not occurring again, recovery can be completed for other cylinders inwhich knocking is not occurring. In this way, it is possible to performthe recovery control securely and stably.

Further, preferably in the present invention, the first recovery unitand the second recovery unit may be configured to start a recoverycontrol if the knocking is not occurring within a predetermined periodafter the halt or reduction of the gas fuel by the knocking reductionunit.

As described above, with the predetermined standby period set afterreduction or halt of the gas fuel by the knocking reduction unit, it ispossible to determine whether the knocking is occurring again during theperiod, which makes it possible to perform the recovery control securelyand stably.

Further, preferably in the present invention, an increase rate of thegas fuel of the first recovery unit may be set to be greater than anincrease rate of the gas fuel of the second recovery unit.

As described above, with the increase rate of the gas fuel of the firstrecovery unit being greater than the increase rate of the gas fuel ofthe second recovery unit, it is possible to shorten the recovery time ofthe first recovery unit as compared to that of the second recovery unit.

Further, the first recovery unit may be set to perform the recoverycontrol to recover after a short period of time, for instance, two tothree seconds after the start. While it is possible to address theknocking effectively by reducing the gas fuel, a decrease in thecombustion temperature in return reduces the temperature inside thecombustion chamber, which may lead to a misfire due to remarkableunstable combustion that emerges when the amount of gas fuel isgradually increased upon recovery. Thus, it is possible to avoid suchunstable recovery by increasing the amount of gas fuel at once.

Further, preferably in the present invention, an amount of reduction ofthe gas fuel reduced by the knocking reduction unit may be set to belarger for the cylinder in which the knocking is occurring than for theother cylinders in which the knocking is not occurring.

For the cylinder in which knocking is occurring, the amount of gas fuelreduction is an amount for shifting the air-fuel ratio to the lean sideto restrict knocking. For other cylinders in which knocking is notoccurring, assuming that there is one cylinder with knocking, the amountof reduction is an amount accompanying the load reduction of the powergeneration output for preventing overload caused by an increase in theload applied to the other cylinder, the increase corresponding to theamount of the one cylinder with knocking.

In this way, the amount of gas-fuel reduction is set to be larger forthe cylinder in which the knocking is occurring than for the othercylinders in which the knocking is not occurring, which makes itpossible to perform the control to reduce knocking effectively.

Further, preferably in the present invention, if the knocking occursagain during the predetermined period, the gas fuel may be reducedfurther by an amount of gas-fuel reduction which is set by the knockingreduction unit at a time when the knocking occurs again.

As described above, if knocking has occurred again during the standbyperiod, it is possible to perform reduction of knocking and thesubsequent recovery control stably by reducing the amount of gas fuelsupply even further at this point of time.

Advantageous Effects

According to the present invention, as a countermeasure to knocking in agas engine, it is possible to prevent the gas engine from entering anunstable combustion state such as misfire upon recovery by halting orreducing the gas fuel for the cylinder in which knocking is occurringand then increasing the amount of gas fuel again to appropriatelyperform recovery control upon recovery to the optimum operation.

Specifically, the first recovery unit recovers the cylinder in whichknocking is occurring from a state in which the gas fuel is halted orreduced, and the second recovery unit recovers cylinders other than thecylinder in which knocking is occurring from a state in which the gasfuel is reduced.

When the amount of gas fuel is increased to perform recovery, therecovery time of the first recovery unit is set to be shorter than therecovery time of the second recovery unit, which makes it possible tocomplete recovery of the cylinder in which knocking is occurring inpriority, and to complete recovery of other cylinders with no knockingafter it is checked whether knocking is occurring again. In this way, itis possible to perform the recovery control securely and stably.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a system diagram of an overall configuration of a combustioncontrol device of a gas engine according to the first embodiment of thepresent invention.

FIG. 2 is a partial cross-sectional explanatory diagram of a peripheralconfiguration of a combustion chamber of the gas engine illustrated inFIG. 1.

FIG. 3 is a flowchart of a combustion control device.

FIGS. 4A to 4C are charts of recovery states of gas fuel recovered by arecovery unit. FIG. 4A is a case of a cylinder other than the cylinderin which knocking is occurring. FIG. 4B is a case of the cylinder inwhich knocking is occurring. FIG. 3 is a comparative example.

FIG. 5 is a chart for explaining a knocking area and a misfire area, thehorizontal axis being an air-fuel ratio X, and the vertical axis beingan engine output P.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings. It is intended, however,that unless particularly specified, dimensions, materials, shapes,relative positions and the like of components described in theembodiments shall be interpreted as illustrative only and not limitativeof the scope of the present invention.

FIG. 1 is a diagram of an overall configuration of a combustion controldevice of a gas engine according to the first embodiment of the presentinvention. FIG. 2 is a partial cross-sectional explanatory diagram of aperipheral configuration of a combustion chamber.

In FIG. 1, the gas engine (hereinafter, merely referred to as “engine”)1 is a multi-cylinder four cycle engine in which gas fuel such asnatural gas and city gas is used as main fuel. A flywheel 3 is mountedto a crankshaft 2 of the engine 1, and a generator 5 is directly mountedto the flywheel 3.

Further, a gas-fuel control device 7 for controlling an amount of gasfuel which is to be supplied into each cylinder of the engine 1, and anignition device 9 for igniting gas fuel supplied into each cylinder areprovided.

As illustrated in FIG. 2, the engine 1 includes a piston 13 reciprocablyfitted in a cylinder 11, a main combustion chamber 17 defined and formedbetween an upper surface of the piston 13 and an inner surface of thecylinder block 15, an intake port 19 connected to the main combustionchamber 17, and an intake valve 21 for opening and closing the intakeport 19.

Further, a gas mixer 25 is disposed in a supply-air pipe 23 at theupstream side of the intake port 19, and a gas supply pipe 27 isconnected to the supply-air pipe 23 at the upstream side of the intakeport 19. Fuel gas supplied through a gas adjustment valve 29 disposed inthe gas supply pipe 27 to adjust the amount of fuel gas and air suppliedthrough the supply-air pipe 23 are premixed in the gas mixer 25.

Then, the premixed mixed gas passes through the intake port 19 to arriveat the intake valve 21, and the intake valve 21 opens, so that thepremix mixed gas is supplied to the main combustion chamber 17.

Here, the supply-air pipe 23, the gas supply pipe 27, the gas adjustmentvalve 29 disposed in the gas supply pipe 27, and the gas mixer 25 areprovided for each cylinder.

Further, an ignition device 9 corresponding to each cylinder is providedfor a cylinder head 31 which forms an upper portion of the maincombustion chamber 17 of the corresponding cylinder. The ignition device9 has a structure including a precombustion chamber (auxiliary chamber)and a spark plug, which are not illustrated. Fuel gas for theprecombustion chamber supplied into the precombustion chamber is ignitedby the spark plug, so that flame produced in the precombustion chamberis injected into the main combustion chamber 17 so as to combust themixed air in the main combustion chamber 17.

The ignition device 9 is configured to be ignited by a spark plug at anappropriate timing on the basis of signals from an engine speed sensor42, a crank-angle sensor 45, and a load sensor 47, and to inject flameinto the main combustion chamber 17.

Further, an exhaust port 35 is connected to the main combustion chamber17, so that exhaust gas after combustion is discharged from the maincombustion chamber 17 when the exhaust valve 37 opens. Further, anexhaust pipe 39 (not illustrated) is connected to the downstream side ofthe exhaust port 35, and an exhaust supercharger (not illustrated) ismounted to the exhaust pipe 39.

Further, a combustion pressure sensor 41 for detecting an in-cylinderpressure in the main combustion chamber 17 is disposed on each cylinder.The engine speed sensor 42 for detecting the engine rotation speed andthe crank-angle sensor 45 are disposed on the flywheel 3. The loadsensor 47 for detecting the load of the generator 5, which is the engineload, is disposed on the generator 5.

The signals from the respective sensors are inputted into the combustioncontrol device 43 described below.

Next, the combustion control device 43 in the engine 1 having the aboveconfiguration will be described.

As illustrated in FIG. 1, the combustion control device 43 includes aknocking determination unit 49 for determining whether an abnormalcombustion state, especially knocking, is occurring in the combustioncondition due to a change in the in-cylinder pressure in the maincombustion chamber 17 of each cylinder mainly on the basis of a signalfrom the combustion pressure sensor 41.

Further, the combustion control device 43 includes a knocking reductionunit 55 which includes the first knocking reduction unit 51 and thesecond knocking reduction unit 53. The first reduction unit 52 halts orreduces supply of gas fuel to the cylinder in which knocking isoccurring and the second knocking reduction unit 53 reduces supply ofgas fuel to cylinders other than the cylinder in which knocking isoccurring, when it is determined that knocking is occurring by theknocking determination unit 49.

The first knocking reduction unit 51 halts or reduces gas fuel for acylinder in which knocking is occurring. Specifically, the gas fuel ishalted or reduced by controlling the opening degree of the gasadjustment valve 29 disposed in the gas supply pipe 27.

For instance, when it is determined that knocking is occurring, areduction rate may be set so that the amount of supply is reduced by 2to 3% from the amount of supply at the time. Further, supply of the gasfuel may be always halted because the reduction control becomes complex.

The above amount of 2 to 3% reduction is the amount of gas fuelreduction which is required to shift the air-fuel ratio to the lean sideto restrict knocking.

Further, the second knocking reduction unit 53 reduces the gas fuel forcylinders other than the cylinder in which knocking is occurring. Inthis case, since knocking is not occurring in the cylinders, the load ofpower generation output is reduced to prevent the load applied to theother cylinders from increasing to cause overload (due to an increase inthe load applied to the other cylinders caused by operation of agovernor mechanism) by the amount corresponding to one cylinder giventhat there is one cylinder in which knocking is occurring, instead ofreduction of gas fuel for shifting the air-fuel ratio to the lean side.

For instance, assuming a case where supply of gas fuel is halted for onecylinder among 18 cylinders due to knocking during operation atapproximately 50% load, the load is reduced by 2 to 3% KW of the powergeneration output and set as an amount of reduction of gas fuelcorresponding to the load reduction.

As described above, the gas fuel is reduced by different amounts betweena cylinder in which knocking is occurring and cylinders other than thecylinder in which knocking is occurring, which makes it possible toachieve an effect to restrict knocking with high efficiency.

Further, the combustion control device 43 includes the first recoveryunit 57 and the second recovery unit 59. The first recovery unit 57recovers to the amount of fuel gas before occurrence of knocking from astate in which the gas fuel is halted or reduced in the cylinder inwhich knocking is occurring, and the second recovery unit 59 recovers tothe amount of gas fuel before reduction of torque from a state in whichgas fuel is reduced in cylinders other than the cylinder in whichknocking is occurring, when it is determined that there is norecurrence, i.e., when it is determined that knocking is not occurringduring a predetermined standby period in a state in which the gas fuelis halted or reduced by the first and second knocking reduction units51, 53.

The combustion control device 43 is configured such that the recoverytime of the first recovery unit 57 is set to be shorter than therecovery time of the second recovery unit 59, so that recovery of thecylinder in which knocking is occurring is performed in priority toother cylinders in which knocking is not occurring.

As described above, recovery is firstly performed for the cylinder inwhich knocking is occurring, and then if abnormality is not occurringagain, i.e., after or while confirming that abnormal combustion such asknocking and misfire is not occurring again, recovery is completed forother cylinders in which knocking is not occurring. In this way, it ispossible to perform the recovery control securely and stably.

Further, reduction and recovery of gas fuel are performed by the firstknocking reduction unit 51, the second knocking reduction unit 53, thefirst recovery unit 57 and the second recovery unit 59 of the combustioncontrol device 43 by controlling the opening degree of the gasadjustment valve 29 constituting the gas fuel control device 7.

Next, with reference to the flowchart of FIG. 3, the control flow of thecombustion control device 43 will be described.

First, when operation is started, an in-cylinder pressure of eachcylinder is detected in step S1. In step S2, knocking is determined anddetermination of occurrence of knocking is repeated until knockingoccurs. If knocking is occurring, the cylinder with knocking isspecified in step S3.

Next, in step S4, the amount of gas fuel is reduced or halted for thecylinder in which knocking is occurring, and the air-fuel ratio isshifted to the lean side (t0 in the time chart of FIG. 4B).Subsequently, in step S5, a standby state is continued for apredetermined time in a condition in which the amount of gas is reducedor halted (t0 to t1 in the time chart of FIG. 4B).

Next, in step S6, it is determined whether knocking has occurred duringthe standby state. If it is determined that knocking is not occurring instep S6, recovery is performed in time to (t1 to t2 in the time chart ofFIG. 4B) in step S7. In the next step S8, it is determined whetherknocking is occurring during the recovery operation.

If knocking is not occurring in step S8, the recovery is completed andthe process is terminated in step S9.

Further, if it is determined that knocking is occurring in step S8, theprocess returns to step S4 to be repeated.

On the other hand, after the cylinder in which knocking is occurring isdetermined in step S3, the amount of gas fuel is reduced in step S10 forother cylinders in which knocking is not occurring (t0 in the time chartof FIG. 4A). Reduction of the amount of gas fuel is set as the amount ofgas fuel reduction which corresponds to the load reduction of 2 to 3% KWin the power generation output for preventing overload.

Then, in step S11, a standby state is continued for a predeterminedperiod of time in a condition in which the amount of gas is reduced (t0to t1 in the time chart of FIG. 4A).

Next, in step S12, it is determined whether knocking has occurred duringthe standby state. If it is determined that knocking is not occurring instep S12, recovery is performed taking time tb (t1 to t3 in the timechart of FIG. 4A) in step S14. In the next step S15, it is determinedwhether knocking is occurring during the recovery operation.

If knocking is not occurring in step S15, recovery is completed and theprocess is terminated in step S16.

On the other hand, if it is determined that knocking is occurring instep S15, the process returns to step S10 to be repeated.

The above steps S10 to S16 have the same process flow as the steps S4 toS9.

Further, in steps S6 and S12, if knocking occurs during the standbystate, the process proceeds to step S13, in which uniform load reductionis performed for all cylinders and the amount of gas fuel is reducedaccordingly.

Recoveries performed by the first recovery unit 57 and the secondrecovery unit 59 are illustrated in FIG. 4.

FIG. 4A is a case of a cylinder other than the cylinder in whichknocking is occurring, in which recovery is performed by the secondrecovery unit 59. FIG. 4B is a case of the cylinder in which knocking isoccurring, in which recovery is performed by the first recovery unit 57.FIG. 4C is a comparative example, in which the amount of supply of thegas fuel is reduced for all of the cylinders along constant loadreduction.

In the comparative example of FIG. 4C, as illustrated in the drawing,uniform load reduction is performed for all of the cylinders, and then,after elapse of the standby time, recovery is performed taking therecovery time tb (in the present invention, longer than the recoverytime ta). Thus, the gas fuel is gradually increased in amount uponrecovery from a state where the temperature in the combustion chamber islow, which makes combustion unstable, and makes misfire likely to occur.

It is possible to avoid such unstable recovery by increasing the amountof gas fuel at once in the recovery time ta (which is shorter than therecovery time tb) as in the present embodiment.

Further, while the first recovery unit 57 and the second recovery unit59 start recovery simultaneously with time t1 as illustrated in FIGS. 4Aand 4B, the second recovery unit 59 illustrated in FIG. 4A may startrecovery at time t2, when recovery by the first recovery unit 57 iscompleted. As described above, with the second recovery unit 59 startingrecovery at time t2, when recovery by the first recovery unit 57 iscompleted, it is possible to confirm that there is no recurrence ofknocking or misfire during recovery in the cylinder in which knocking isoccurring, and to perform a recovery control stably and securely byrecovering cylinders other than the cylinders in which knocking isoccurring after the confirming.

FIG. 5 is a chart of a region in which abnormal combustion (e.g.knocking or misfire) of the engine 1 occurs. The horizontal axis is anair-fuel ratio and the vertical axis is an engine output P.

The engine 1 of the present embodiment performs high-output operation,and is operated at substantially constant rotation at point P1. Theoperation point is likely to shift to the operation point P2 in theknocking area due to a change in the power generation load or in theenvironmental conditions. If the operation point has moved to theknocking area, it is necessary to reduce the output or shift theair-fuel ratio to the lean side to return the operation point to thenormal operation area from the knocking area.

In the present embodiment, knocking is avoided by reducing the amount ofgas fuel at the operation point by 2 to 3% for the cylinder in whichknocking is occurring and by shifting the air-fuel ratio to the leanside.

According to the above embodiment, gas fuel is halted or reduced for thecylinder in which knocking is occurring as a countermeasure to knockingin the engine 1, and then the gas fuel is appropriately increased in thesubsequent recovery control to the optimum operation, which makes itpossible to prevent the engine 1 from entering an unstable combustionstate such as misfire upon recovery.

Specifically, the first recovery unit 57 recovers the cylinder in whichknocking is occurring from a state in which the gas fuel is halted orreduced, and the second recovery unit 59 recovers cylinders other thanthe cylinder in which knocking is occurring from a state in which thegas fuel is reduced.

Upon this recovery, the recovery time of the first recovery unit 57 isset to be shorter than the recovery time of the second recovery unit 59so as to recover the cylinder in which knocking is occurring as apriority, and then completes recovery of other cylinders with noknocking. Thus, it is possible to perform the recovery control securelyand stably.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to prevent a gasengine from entering an unstable combustion state such as misfire byhalting or reducing gas fuel for a cylinder in which knocking isoccurring as a countermeasure to knocking in the gas engine and thenincreasing the amount of gas fuel again to appropriately performrecovery control upon recover to the optimum operation. Thus, thepresent invention can be suitably applied to a combustion control deviceof a gas engine for a generator.

DESCRIPTION OF REFERENCE NUMERALS

-   1 Engine (gas engine)-   5 Generator-   7 Gas fuel control device-   9 Ignition device-   41 Combustion pressure sensor-   42 Engine speed sensor-   43 Combustion control device-   45 Crank angle sensor-   47 Load sensor-   49 Knocking determination unit-   51 First knocking reduction unit-   53 Second knocking reduction unit-   55 Knocking reduction unit-   57 First recovery unit-   59 Second recovery unit

1. A combustion control device for a gas engine, comprising: a knockingdetermination unit configured to determine occurrence of knocking ofeach of cylinders; a knocking reduction unit configured to halt orreduce supply of gas fuel to a cylinder in which the knocking isoccurring and reduce supply of the gas fuel to other cylinders in whichthe knocking is not occurring, when the occurrence of the knocking isdetermined by the knocking occurrence determination unit; a firstrecovery unit configured to recover a state where the gas fuel is haltedor reduced in the cylinder in which the knocking is occurring, when itis determined that the knocking is not occurring after the halt orreduction; and a second recovery unit configured to recover a statewhere the gas fuel is reduced in the other cylinders which are not thecylinder in which the knocking is occurring, wherein a recovery time ofthe first recovery unit is shorter than a recovery time of the secondrecovery unit, and recovery of the cylinder in which the knocking isoccurring is performed as a priority.
 2. The combustion control devicefor a gas engine according to claim 1, wherein the first recovery unitand the second recovery unit are configured to start a recovery controlif the knocking is not occurring within a predetermined period after thehalt or reduction of the gas fuel by the knocking reduction unit.
 3. Thecombustion control device for a gas engine according to claim 1, whereinan increase rate of the gas fuel of the first recovery unit is set to begreater than an increase rate of the gas fuel of the second recoveryunit.
 4. The combustion control device for a gas engine according to anyone of claims 1 to 3, wherein an amount of reduction of the gas fuelreduced by the knocking reduction unit is set to be larger for thecylinder in which the knocking is occurring than for the other cylindersin which the knocking is not occurring.
 5. The combustion control devicefor a gas engine according to claim 2, wherein, if the knocking occursagain in the predetermined period, the gas fuel is reduced further by anamount of reduction of the gas fuel which is set by the knockingreduction unit at a time when the knocking occurs again.